I have a 2003 Ford Focus that will not start.

I have a 2003 Ford Focus that will not start. Seems to be a fuel issue, but not sure. The owner had replaced the plugs, cap before bring it to me. The head is new and has a new timing belt as well as a new fuel pump. Turns over fine, no codes but will not fire. Occationally sounds like it is trying to backfire thorugh the exhaust but was doing this before the head change and the timing belt.